This is one of the main sources of experience in Arcanum. There are thousands of monsters in various areas scattered throughout Arcanum and killing them rewards a lot of experience. For each successful hit against a monster you will get some experience. This means the more hits you do, the more experience you get. This also means that if you have sidekicks/followers that participate in combat, they will steal some of that experience.

Almost all of the quests in Arcanum will offer you experience as part of the reward. To level up faster, make sure you try to complete as many side-quests as possible in each of the city areas. You can find out about quests by talking to every single person in the city and choosing different dialogue options or you can use this wiki to find out the quest locations and solutions.

Go to the Forbidden Pit at location 830W 1035S. Enter the pit and kill the 3 spiders there. You can wait for more monsters by waiting one day. Keep repeating the method until whatever level you desire.

A much faster & efficient way to level up is by killing monsters that come through the portal in Find Liam Cameron quest. The monsters spawning through the portal will let you level up extremely fast. However, the monsters are very strong so make sure you can kill them fast.

If you travel from location to location in the overworld, more often than not you will encounter random enemies which will attack you on sight. You can simply keep travelling around in the overworld endlessly and kill the monsters you encounter.

Arcanum has a level 50 cap in the regular version. This level is easily reached by the middle of the game if you do a lot of quests and explore the world. There is a mod available that allows you to set a manual level cap between 50-127 to your choosing. The mod is AVAILABLE HERE.